Weird.
client: OS X 10.5 firefox 3 b5.
I'm running the very same configuration (I think that's the one that matters
since it's probably a layout / css / js issue) on a local XWiki instance and
everything's working fine... Do you have Firebug enabled? If you do, do you
see a javascript error of any kind ?
xwiki is in a tomcat container (v. 5.5.12) and the
java version is 1.5
Guillaume